1. Field of the Invention
This invention relates to a technique for reducing flicker of image shot with a video camera caused by luminance fluctuation of an illuminating light source.
2. Description of the Related Art
The frequency of commercial electrical power is standardized at either 50 Hz or 60 Hz depending on geographical region. Depending on the type of lighting equipment, the luminance of a lighting equipment which uses such commercial electrical power fluctuates at a frequency equal to the frequency of commercial electrical power (commercial power frequency) or a twice of the commercial power frequency. For example, in regions of 50 Hz commercial power frequency, the luminance of an incandescent lamp fluctuates at 100 Hz, while the luminance of a fluorescent lamp without an inverter fluctuates at 100 Hz or 50 Hz.
Meanwhile, the number of frames per second (frame rate) in a video camera and a video monitor is standardized for the television system. For example, in the case of the NTSC television system, the frame rate is standardized to be 60 frames per second (FPS).
For example, in the case of using a video camera with a frame rate of 60 FPS in a region of 50 Hz commercial power frequency to shoot video image of a subject illuminated by fluorescent light, the brightness fluctuation of individual frames, a phenomenon known as flicker, occurs resulting from the lightness fluctuation of the subject caused by the luminance fluctuation of the lighting equipment causes. Typically, the video camera is equipped with flicker reduction means for reducing such flicker. In an example of such flicker reduction means, the shutter speed is set to 1/00 second in order to reduce variation of light amount entering into the imaging elements of the video camera.
However, in such flicker reduction means provided to a video camera, flicker reduction processing is fixed for each model of the video camera. For this reason, it may sometimes occur that, flicker is not sufficiently reduced with a certain video camera under the conditions of the illuminating light source, while the picture quality of the video image output from a different video camera may degrade due to an excessive flicker reduction.